1. Field of the Invention
The present invention relates to a process for the restrained drying of a paper web in a dryer section of a paper machine. More particularly, the present invention relatives to a process which includes restraining the web against cross-machine direction shrinkage so that edge curl of the web is inhibited.
2. Information Disclosure Statement
In the manufacture of a paper web, when a web has been formed, it is thereafter pressed in a press section to remove a substantial portion of the moisture therefrom. The web is then guided through a dryer section in order to remove a further portion of moisture from the web so that the resultant web will attain the required sheet characteristics.
A typical dryer section includes a plurality of dryers arranged as a first and a second tier with the first tier disposed above the lower tier. The web is threaded around alternate upper and lower dryers in serpentine configuration so that alternate sides of the web are sequentially exposed to the external surfaces of the dryers.
In the aforementioned arrangement, the upper tier of dryers is wrapped by an upper dryer felt and the lower tier of dryers is wrapped by a lower dryer felt. The arrangement is such that the upper and lower felts restrain the web against cross-machine direction shrinkage during passage of the web around the respective upper and lower dryers. However, a problem exists in that with the aforementioned arrangement, the web is unsupported by either the upper or the lower felt during transit of the web between the upper and lower dryers. Such unrestrained movement of the web permits cross-machine direction shrinkage with the attendant resultant edge curl.
In an attempt to reduce edge curl and cockle in the resultant web, dryer sections have been installed in which a single dryer felt extends contiguously with the web around respective upper and lower dryers so that no open draw or unsupported passage of the web between dryers is permitted. While the aforementioned single felted or Uno-run dryer sections assist in supporting the web during transit between upper and lower dryers such single felted arrangements suffer from at least the following disadvantages. First, the felt is disposed between the web and the lower dryer thereby rendering the lower dryers redundant. Second, the web is disposed on the outside of the felt during passage around the lower dryers so that there exists a tendency for the web to flutter relative to the felt during passage around the lower dryers. Such fluttering of the web means that the web is unrestrained during passage around the lower dryer drum.
Ideally, the web should be restrained against cross-machine direction shrinkage during the entire passage of the web through the dryer section. Such ideal situation is achieved by the employment of the TOTAL BEL RUN arrangement proposed in copending patent application Ser. No. 014,569 filed Feb. 13, 1987 and assigned to Beloit Corporation.
In Ser. No. 014,569 there is disclosed a single tier dryer section in which the web and felt disposed contiguously relative to each other are guided alternately around dryers and vacuum guide rolls. The guide rolls replace the lower redundant dryer drums of the aforementioned single felt arrangements. The vacuum guide rolls draw the web towards the guide roll during passage of the web around the guide roll such that the web is pressed against the felt during passage around the vacuum guide roll thereby restraining the web against cross-machine direction shrinkage.
Therefore, cross-machine direction shrinkage is not only inhibited during passage of the web around the dryer but also around the vacuum guide roll. Furthermore, the vacuum guide roll being of a diameter considerably less than that of the dryer results in a joint run of the web and felt between the dryers and the guide rolls which is minimal so that the web is restrained against cross-machine direction shrinkage throughout most of the passage through the dryer section.
The aforementioned sheet restraint reduces edge curl and cockle and the graininess of the resultant sheet at the edges thereof. Furthermore, by the provision of such sheet restraint, the slice opening in the headbox is able to be more uniform and the cross direction fiber orientation profile is improved.
More specifically, various laboratory and mill studies have been carried out in order to quantify the nonuniform cross directional sheet shrinkage which occurs during conventional drying processes. The aforementioned nonuniform shrinkage is responsible for nonuniformities in headbox slice profiles, in fiber orientation, and in the sheet elongation and tensile energy absorption.
Tensile energy absorption hereinafter referred to as TEA is defined in "The Dictionary of Paper" Fourth Edition, published 1980, as the energy absorbed when a paper specimen is stressed to rupture under tension. It is expressed in energy units per unit area e.g. kg-cm/cm.sup.2. It is useful in evaluating packaging materials subject to rough handling.
The continuous drying restraint which is applied by a total no-draw dryer section as exemplified in the TOTAL BEL RUN concept of Beloit Corporation, has a direct effect on the finished sheet properties by controlling the cross-directional elongation and TEA profiles. Additionally, such reduced shrinkage reduces the cockles and graininess of the sheet edges.
The aforementioned dictionary defines cockles as "a puckered condition of the sheet resulting from nonuniform drying and shrinking; it usually appears on paper that has had very little restraint during drying."
Furthermore, graininess is defined in the aforementioned dictionary as small variations in the surface appearance of a paper or board, resulting from any of a variety of causes, such as impressions of wires or felts, irregular distribution of color, and uneven shrinkage in drying.
Also, by restraining the sheet from cross direction shrinkage, the opening of the slice lip of the headbox may be maintained more uniform and an improved cross direction fiber orientation profile is obtained as stated more particularly hereinafter.
The aforementioned TOTAL BEL RUN includes the transfer of the web between dryers with positive support and restraining the sheet with fabric pressure and roll vacuum. The combination of the aforementioned arrangement has improved sheet threading, machine runability, and sheet properties.
In a conventional dryer section, the wet paper is dried by intermittent contact with cast-iron, steam heated dryers. The thermal contact between the paper and the dryer is maintained by tensioned dryer fabrics which apply a pressure to the paper as it wraps the dryer. Typical fabric tensions range from 8 to 12 pounds per linear inch (PLI) which depending on the dryer diameter, will apply a fabric pressure which is in the range of 0.25 to 0.35 PSI which is 6 to 10 inches water gauge (WG).
The aforementioned fabric pressure not only improves the drying contact, but also applies a restraint to the paper to prevent shrinkage from occurring. Such restraint, however, is repeatedly released as the sheet passes through the open draws between conventional dryer cylinders as described hereinbefore.
The fabric pressure continues to provide some restraint in the machine direction by maintaining a machine direction draw, but in the cross-machine direction, the paper is virtually unrestrained. The paper shrinks freely in the cross direction particularly at the edges and somewhat less so near the center of the web where the sheet is at least partly restrained by the outer portions.
Such nonuniform cross-machine shrinkage gives rise to nonuniform cross-directional sheet properties such as stretch, TEA and tensile.
Stretch is defined in the aforementioned dictionary as "the elongation corresponding to the point of rupture in a tensile strength measurement; it is usually expressed as a percentage of the original length."
The high cross-directional edge shrinkage also aggravates the susceptibility of the sheet to edge cockle, curl and graininess.
The aforementioned dictionary defines curl as "the curvature developed when one side of a paper specimen is wetted; it was formally used as a measure of the degree of sizing."
The lack of shrinkage restraint also increases the hygroexpansivity and can also have an adverse effect on fiber orientation. Hygroexpansivity is defined in "The Dictionary of Paper" as "the change in dimension of paper that results from a change in the ambient relative humidity; it is commonly expressed as a percentage and is usually several times higher for the cross direction than for the machine direction. This property is of great importance in applications where the dimensions of paper sheets and cards or construction board (wallboard, acoustical tile, etc.) are critical."
In various mill trials, the first phase of such study was directed at quantifying the nonuniformity on commercial paper machines and then determining the effect that the nonuniform shrinkage has on the machine operation and on the finished sheet properties.
The cross-directional sheet shrinkage was determined by metering fine drops of ink onto the stock as it discharged from the slice lip of the headbox. The distances between marks at the wet end were then compared to the distances at the dry end to determine the cross-directional shrinkage profile.
Results for a fine paper machine are discussed hereinafter. The shrinkage was found to be highly nonuniform, and in fact almost parabolic. As expected, the highest shrinkage was found to occur at the edges, where the sheet has the least cross-directional restraint and the sheet shrinkage was the lowest near the center where the paper was at least partly restrained by the outer portions.
A cross-directional paper sample was then tested in the laboratory to determine the variations in sheet properties and these results are discussed in greater detail hereinafter. Such results show the machine direction stretch is very uniform in the cross direction because it is controlled by the machine direction draws. However, the cross-directional stretch is very nonuniform which appears to be a direct reflection of the cross-directional shrinkage. In other words, the highest stretch occurs at the edges where the sheet has experienced the greatest shrinkage.
The machine direction and cross direction tensile strength profiles were also measured for the same sample.
Tensile strength is defined in the aforementioned dictionary as "the maximum tensile stress developed in a specimen before rupture under prescribed conditions; it is usually expressed as force per unit width of the specimen."
As discussed hereinafter the machine direction tensile was fairly uniform, again being affected in part by the machine direction draw which does not vary in the cross direction. However, the cross direction tensile profile is nonuniform and exhibits a slight hyperbolic configuration. The lowest tensile occurs near the sheet edges, again where the cross-direction shrinkage was the greatest.
From the aforementioned tests, it is also evident that an increase in cross-direction restraint, as experienced near the center of the machine, causes a reduction in stretch with a corresponding increase in tensile strength. Since the cross-direction tensile varies in the cross direction, while the machine direction tensile remains fairly uniform, the tensile ratio also varies, with the highest ratio occurring at the edges.
The tensile ratio is the ratio of the tensile in the cross direction to the tensile in the machine direction and will be discussed in detail hereinafter.
The TEA profiles were also measured for the sample. The cross-direction profile reflected the nonuniformity in cross-direction stretch. The TEA profile, however, does not exhibit quite as much variation as the CD stretch, because the loss in stretch near the machine center is greatly offset by the increase in tensile strength.
The increased shrinkage which occurs near the edges also has an adverse effect on headbox performance. In order to produce a level basis weight profile at the reel, the slice opening must be closed down near the edges. Such closing down near the edges of the slice opening reduces the basis weight at the edges to compensate for the higher shrinkage which occurs near the edges. Such reduction in basis weight causes the paper to go through the press section and earlier dryer sections with light edges which eventually heavy up as the edges shrink.
Basis weight is defined in the aforementioned dictionary as "the weight in pounds of a ream cut to a specified basis size. The number of sheets in a ream is usually 500."
The aforementioned nonuniform slice opening is known to cause a distortion of the fiber orientation by inducing cross flows.
The fiber orientation was determined for the aforementioned sample by measuring the sonic modulus profile as discussed hereinafter. The fiber orientation is indicated as the angle of the primary axis of the modulus envelope from the machine direction. A positive angle indicates the fibers are oriented towards the back side of the web and a negative angle indicates the fibers are oriented towards the front side.
The fibers are all oriented towards machine center line as would be expected because the slice opening is closed down near the edges to compensate for edge shrinkage.
The aforementioned advantages obtained by restrained drying of the web are reflected in considerable commercial advantages over webs produced in nonrestrained drying sections.
In order to achieve a level weight profile with a uniform slice opening, it is necessary to control the cross-direction shrinkage. Since this shrinkage occurs as the moisture is removed, the majority of the shrinkage takes place in the open draws where water flashes. In order to reduce the shrinkage, the open draws must be replaced by means of positive restraint.
A common commercial arrangement for eliminating open draws is the single felt or serpentine dryer section. Although such serpentine arrangement does eliminate the open draws, it does not replace the open draws with positive restraint and it dries the sheet from one side only.
The aforementioned TOTAL BEL RUN, however, replaces the bottom ineffective dryers of the serpentine section with vacuum rolls. Two-sided drying is maintained in this arrangement by alternating between top-felted and bottom-felted single tier sections as described in the aforementioned pending patent application Ser. No. 014,569.
The intermediate vacuum roll of the aforementioned single tier section acts much like the fabric vacuum box used in laboratory studies discussed in detail hereinafter. The vacuum maintains the restraint which is applied by the dryer fabric pressure as the sheet is transferred between dryers.
The vacuum induced in conventional serpentine blow boxes is typically only 0.1 to 0.2 inches water column (WC) and this is clearly inadequate to provide significant shrinkage restraint. Additionally, such low level vacuum does not extend around the entire bottom dryer. With the longer sheet length between top dryers, the sheet is left unrestrained for a significant portion of the drying cycle in the conventional serpentine or single felt dryer section.
A vacuum level of 6 to 8 inches water column in the vacuum rolls is essentially equal to the restraint which is applied by the dryer fabric. Such vacuum level is also the level which is required for positive sheet restraint as discussed hereinafter.
The sheet restraint used in the various laboratory studies was applied continuously and in order to achieve the same property improvements on a commercial machine, the drying restraint was also applied continuously or at least in those sections where the sheet is shrinking the most. Very specific laboratory tests were carried out on pilot machine samples in order to determine the natural or unrestrained shrinkage characteristics.
For the furnish used, the machine direction and cross-direction shrinkage is very low as the sheet is dried from 40 to 60 percent dry. Once the sheet reaches 60 percent dry, the shrinkage increases and continues at a high rate until the sheet is essentially dry.
The serpentine and in some cases single-tier dryer sections had been used at the wet end of the dryer section. Such application had been carried out to improve runability. However, in view of the aforementioned increased shrinkage above 60 percent dry, it became evident that the single-tier dryer section should be applied near the dry end of the machine for improved paper properties and for the best runability and sheet quality, the single-tier dryer section configuration should be applied to the entire dryer section as taught in the aforementioned pending patent application Ser. No. 014,569.
Therefore it is a primary object of the present invention to provide a process that overcomes the aforementioned inadequacies of the prior art drying methods and to provide a process which makes a considerable contribution to the art of paper drying.
Another object of the present invention is the provision of a process for restrained drying of a paper web in a dryer section which includes the steps of wrapping the web with a felt during passage of the web with the felt around the dryer of the dryer section and thereafter wrapping the web around a portion of a guiding device disposed immediately downstream relative to the dryer so the edge curl of the web is minimized.
Another object of the present invention is the provision of a process which includes restraining the web against cross-machine direction shrinkage during passage of the web past the guiding device so that edge curl of the web is inhibited.
Another object of the present invention is the provision of a process for the restrained drying of a paper web which reduces cockle and graininess of the sheet edges.
Another object of the present invention is the provision of a process which allows the slice opening to be more uniform thereby improving the cross-direction fiber orientation profile.
Other objects and advantages of the present invention will be evident from the detailed description contained hereinafter taken in conjunction with the various figures of the drawings and graphs and from the disclosure of the appended claims.